1. Field of the Invention
The present invention relates to a color cathoderay tube in which a phosphor screen is formed on the inner surface of a flat face plate, and a method of manufacturing the same.
2. Description of the Related Art
Recently, various researches have been made on high-definition broadcasting and a high-resolution cathode-ray tube with a large screen designed for the high-definition broadcasting. In general, in order to achieve high resolution of a cathode-ray tube, the spot diameter of an electron beam on a phosphor screen must be reduced.
For this purpose, in the prior art, the structure of an electrode of an electron gun has been improved, or the caliber and/or length of the electron gun has been increased. However, satisfactory achievement has not been obtained. The main reason for the failure is that the distance between the electron gun and the phosphor screen increases in accordance with increases in the size of the tube and increases in the magnification of the electron lens. Accordingly, in order to achieve high resolution, it is important to shorten the distance (depth) between the electron gun and the phosphor screen. In addition, when the wide-angle deflection is used, the difference in magnification between the center area and peripheral area of the phosphor screen increases. Thus, the wide-angle deflection is not advantageous for achieving high resolution.
Jpn. Pat. Appln. KOKAI Publication No. 48-90428 discloses a method of arranging a plurality of independent small-sized cathode-ray tubes, thereby constituting a high-resolution, large screen. This kind of method is effective for large-scale screen display with a large number of divided regions, which is designed for outdoor installation. However, when this method is applied to middle-scale screen display (e.g., the screen size is about 40 inches), connection portions between the divided regions of the screen are conspicuous, resulting in low-quality images. Thus, when the display formed by this method is used a household TV receiver or computer-aided design (CAD), the connection portions on the screen are a serious defect.
On the other hand, U.S. Pat. No. 3,071,706 or the like discloses a structure wherein phosphor screens of a plurality of independent cathode-ray tubes are integrated. In this cathode-ray tube having the integrated phosphor screen, a vacuum envelope is constituted by a face plate on which a phosphor screen is coated, a rear plate opposed and arranged to the face plate, and a plurality of funnels adjacent to the rear plate.
With this structure, however, if the screen surface becomes broader, it is necessary to increase the thickness of the face plate or rear plate in order to withstand the load of atmospheric pressure (external pressure). In addition, it is necessary to provide a face plate with a high curvature in the tube axis direction. As a result, the weight of the envelope becomes considerably heavy. Moreover, when the high curvature of the face plate increases, the screen cannot be viewed clearly. In addition, the distance between the phosphor screen and the electron gun sealed within the neck increases, and the magnification of the electron lens is adversely affected.
In order to solve the problems posed in the cathode-ray tube having the above integrated phosphor screen, Jpn. Pat. Appln. KOKAI Publication No. 5-36363 discloses a cathode-ray tube (color cathode-ray tube) in which a face plate is formed to be flat, and an integrated phosphor screen formed on the inner surface of the face plate has a plurality of regions which are scanned independently of one another by electron beams emitted from a plurality of electron guns. In this cathode-ray tube, plate support means are arranged inside an envelope between a face plate and a rear plate to support the load of atmospheric pressure acting on the flat face plate and the flat rear plate opposing the face plate.
Furthermore, U.S. patent application Ser. No. 945,415, filed Sep. 16, 1992 discloses a cathode-ray tube (color cathode-ray tube) in which a plate support member for supporting the load of atmospheric pressure acting on a flat face plate, a flat rear plate opposing this face plate, and mask mounting means for supporting a shadow mask are fixed to a fixing member which is fixed in tight contact with the inner surface of the rear plate.
Such a color cathode-ray tube is manufactured in the following manner. A phosphor screen is coated on the inner surface of a face plate in advance. Plate support means and mask mounting means are fixed to a rear plate, and a shadow mask is mounted on the mask mounting means. Then, the face plate on which the phosphor screen is formed is joined to the rear plate, on which the plate support means and the shadow mask are mounted, through a side wall. Therefore, it is difficult to assemble the phosphor screen and the shadow mask in a predetermined relationship with high precision.
More specifically, the phosphor screen of the color cathode-ray tube has stripe-shaped three color phosphor layers which extend in the vertical direction in parallel with one another and are arranged in the horizontal direction. This phosphor screen is manufactured by using a master mask on which reference patterns are formed at predetermined pitches. Specifically, the pattern of the master mask are formed on a screen formation material layer coated on the inner surface of the face plate by a photoprinting method. Therefore, in assembly of the above color cathode-ray tube, the phosphor screen should be accurately positioned at a predetermined position with respect to the shadow mask mounted on the rear plate via the mask mounting means, thereby positioning the face plate.
The phosphor screen must be accurately positioned in the horizontal, vertical, and rotational directions of the three stripe-like color phosphor layers with respect to the shadow mask. The required precision of this positioning is about 10% or less of the width of the three color phosphor layers, though it depends on the pitches of the three color phosphor layers. Higher precision are required particularly in the horizontal and rotational directions. If one end of the phosphor screen in the horizontal direction is defined as a reference, an offset at the other end in the horizontal direction must be 0.01 mm or less.
Still further, in the structure in which the shadow mask is mounted on the rear plate via the mask mounting means in the above manner, the phosphor screen is indirectly positioned with respect to the shadow mask via the mask mounting means, the rear plate, the side wall, and the face plate. For this reason, even if the precision of an assembly jig used in the assembly process is improved, a cumulative error further increases. Thus, it is difficult to obtain desired precision.